1. Field of the Invention
The present invention relates to the improvements of an intake-air quantity control apparatus for an internal combustion engine equipped with a variable valve timing system capable of electronically arbitrarily controlling an intake- and/or exhaust-valve timing depending on engine/vehicle operating conditions, and specifically to techniques for controlling engine power output (a quantity of intake air entering an internal combustion engine) by adjusting an intake-valve open timing (often abbreviated to xe2x80x9cIVOxe2x80x9d) and an intake-valve closure timing (often abbreviated to xe2x80x9cIVCxe2x80x9d).
2. Description of the Prior Art
In recent years, there have been proposed and developed various electronically-controlled variable valve timing systems which are capable of operating intake and exhaust valves electromagnetically. One such electronically-controlled variable valve timing system for an internal combustion engine having electromagnetically-powered valve units has been disclosed in Japanese Patent Provisional Publication No. 10-311231. In the Japanese Patent Provisional Publication No. 10-311231, each of intake and exhaust valves is comprised of an electromagnetic solenoid valve whose opening and closing are achieved by way of an electromagnetic force instead of the use of a typical cam-drive mechanism. Thus, an intake-valve open timing (IVO), an intake-valve open timing (IVO), an exhaust-valve open timing (EVO), and an exhaust-valve closure timing (EVC) can be continually changed in response to command signals from an electronic control module (ECM). In such internal combustion engines with a variable valve timing control system having electromagnetically-powered valve units, an intake-air quantity can be adjusted by properly controlling or managing an intake valve timing (IVO and/or IVC), in place of throttle-opening adjustment. In this type of engines with electromagnetically-powered engine valve units, a throttle valve is often eliminated, or a throttle valve is installed on the engine only for the purpose of generation of a negative pressure in an intake-air passage. Suppose an internal pressure in the intake-air passage reaches a pressure level close to atmospheric pressure with the throttle kept at an extremely less throttle opening. In this case, the intake-air quantity control system based on adjustment of an intake-valve opening time period (a time interval between IVO and IVC) is superior to that based on only the throttle-opening adjustment, from the viewpoint of reduced pumping loss and reduced fuel consumption rate.
In the previously-described internal combustion engine with a variable valve timing system capable of electronically arbitrarily controlling an intake- and/or exhaust-valve timing, when the engine is operated under a particular condition in which an internal pressure in an intake-air passage is kept at a pressure level substantially corresponding to atmospheric pressure, there is a tendency for a flow velocity of air fuel mixture drawn into the engine to reduce as compared to an internal engine utilizing the throttle-opening adjustment to provide intake-air quantity control. This lowers gas flow (in-cylinder mixture flow) in the combustion chamber, thus lowering the combustion stability. Each car now has an evaporative emission control system as one of automotive emission control systems. This is a system that captures or traps any fuel vapors coming from a fuel tank and prevents them from escaping into atmosphere. A typical evaporative emission control system for an internal combustion engine, has a carbon or charcoal canister filled with activated carbon or charcoal for temporarily storing, trapping or adsorbing fuel vapors emitted from a fuel tank, and a purge control valve disposed in a purge line connecting an induction system with the canister. Generally, the action of clearing or removing the trapped fuel vapor from the canister is called xe2x80x9cpurgingxe2x80x9d. Usually, when predetermined engine operating conditions are satisfied after the engine is started, the purge control valve is opened and thus engine vacuum (negative pressure) is admitted to the canister. Thus, the engine vacuum draws fresh air up through the canister via an air port. The fresh air flowing through the interior of the canister, picks up these trapped fuel vapors, and removes the trapped fuel vapors from the canister, and thereafter the purge gas is burned in the combustion chamber. As discussed above, the negative pressure for xe2x80x9cpurgingxe2x80x9d is necessary. For this purpose, it is advantageous to provide an intake-air throttle valve (simply, a throttle) in an intake-air passage. Under a first specified condition where a combustion quality deteriorates, for example, during cold engine operation, the intake-air quantity control system (hereinafter referred to as a xe2x80x9cfirst control mode systemxe2x80x9d) based on throttle-opening adjustment is effective, because it is possible to increase the flow velocity of air-fuel mixture while maintaining the internal pressure in the intake-air passage at a predetermined negative pressure level by controlling or managing the intake-air quantity by means of the throttle. The intake-air quantity control system based on throttle-opening adjustment enhances the combustion quality under the first specified condition. On the other hand, under a second specified condition where the combustion quality (combustion stability) of the engine is good, for example, after engine warm-up, the intake-air quantity control system (hereinafter referred to as a xe2x80x9csecond control mode systemxe2x80x9d) based on adjustment of an intake-valve opening time period (a time interval between IVO and IVC) and/or adjustment of an exhaust-valve opening time period (a time interval between EVO and EVC) is effective. This is because it is possible to lower the fuel consumption rate by executing the second control mode based on adjustment of the intake-valve opening time period and/or adjustment of the exhaust-valve opening time period while maintaining the internal pressure in the intake-air passage at a pressure level substantially corresponding to atmospheric pressure with the throttle held at an extremely less throttle opening. However, in an electronically-controlled engine which is switchable between the first and second control modes during operation of the engine, there is a tendency for a difference in engine power output (engine output torque) to occur during switching between the first and second control modes. This deteriorates vehicle driveability. Such a difference in engine power output is caused mainly by the following two factors. First, the pumping loss and combustion efficiency given during the first control mode are different from those given during the second control mode, and thus there is a difference in a quantity of air required to obtain a desired engine power output, between the first and second control modes. Additionally, during switching between the first and second control modes, the internal pressure in the intake-air passage tends to fluctuate or vary transiently, and therefore a required air quantity also fluctuates transiently. Second, a response characteristic of the first control mode system are different from that of the second control mode system. In the first control mode system (the intake-air quantity control system based on adjustment of the intake-valve opening time period and/or adjustment of the exhaust-valve opening time period), a volumetric capacity from the throttle to the intake valve acts as a time-delay element, and thus an actual intake-air quantity (the value of the controlled quantity) is brought closer to a desired intake-air quantity with a time delay. On the other hand, in the second control mode system (the intake-air quantity control system based on throttle-opening adjustment), it is possible to bring the actual intake-air quantity closer to the desired intake-air quantity without any time delay.
Accordingly, it is an object of the invention to provide an intake-air quantity control apparatus for an internal combustion engine with a variable valve timing system, which avoids the aforementioned disadvantages of the prior art.
It is another object of the invention to provide an intake-air quantity control apparatus for an internal combustion engine with a variable valve timing system, which is capable of achieving smooth switching between a first control mode (an intake-air quantity control mode based on at least adjustment of an intake-valve opening time period) and a second control mode (an intake-air quantity control mode based on throttle-opening adjustment) without any torque difference during operation of the engine.
In order to accomplish the aforementioned and other objects of the present invention, an intake-air quantity control apparatus for an internal combustion engine with a variable valve timing system comprises a throttle valve disposed in an intake-air passage of the engine and controlled so that a throttle opening of the throttle valve is brought closer to a target throttle opening, an intake valve disposed between the intake-air passage and a combustion chamber of the engine and controlled so that an intake-valve closure timing of said intake valve is brought close to a target intake-valve closure timing, and a microprocessor programmed to perform the following:
selecting one of a first control mode in which an intake-air quantity of the engine is controlled by adjusting the throttle opening of the throttle valve, and a second control mode in which an intake-air quantity of the engine is controlled by adjusting the intake-valve closure timing of said intake valve;
calculating a steady-state target engine torque based on operating conditions of the engine, the steady-state target engine torque indicating a steady-state target value of engine torque;
calculating a target engine torque based on the steady-state target engine torque, the target engine torque following the steady-state target engine torque with a predetermined time delay;
setting a steady-state target intake-valve closure timing at a basic intake-valve closure timing when the first control mode is selected, the steady-state target intake-valve closure timing indicating a steady-state target value of the intake-valve closure timing;
calculating a steady-state target intake-air-passage internal pressure based on both the steady-state target engine torque and the steady-state target intake-valve closure timing when the first control mode is selected, the steady-state target intake-air-passage internal pressure indicating a steady-state target value of an internal pressure in the intake-air passage;
setting the steady-state target intake-air-passage internal pressure at a basic pressure when the second control mode is selected;
calculating the steady-state target intake-valve closure timing based on both the target engine torque and the steady-state target intake-air-passage internal pressure when the second control mode is selected;
obtaining a real intake-air-passage internal pressure, the real intake-air-passage internal pressure indicating an actual internal pressure in the intake-air passage;
calculating the target throttle opening based on both the steady-state target intake-valve closure timing and the steady-state target intake-air-passage internal pressure; and
calculating the target intake-valve closure timing based on both the target engine torque and the real intake-air-passage internal pressure.
According to another aspect of the invention, an intake-air quantity control apparatus for an internal combustion engine with a variable valve timing system comprises a throttle valve disposed in an intake-air passage of the engine and controlled so that a throttle opening of the throttle valve is brought closer to a target throttle opening, an intake valve disposed between the intake-air passage and a combustion chamber of the engine and controlled so that an intake-valve closure timing of the intake valve is brought close to a target intake-valve closure timing, and a microprocessor programmed to perform the following:
selecting one of a first control mode in which an intake-air quantity of the engine is controlled by adjusting the throttle opening of the throttle valve, and a second control mode in which an intake-air quantity of the engine is controlled by adjusting the intake valve closure timing of the intake valve;
calculating a steady-state target intake-air quantity based on operating conditions of the engine, the steady-state target intake-air quantity indicating a steady-state target value of intake-air quantity needed when the first control mode is selected;
calculating a target intake-air quantity based on the steady-state target intake-air quantity, the target intake-air quantity indicating a target value of intake-air quantity needed when the second control mode is selected;
calculating the target throttle opening based on the steady-state target intake-air quantity when the first control mode is selected;
setting the target intake-valve closure timing at a basic intake-valve closure timing when the first control mode is selected;
setting the target throttle opening at a predetermined throttle opening when the second control mode is selected, the predetermined throttle opening indicating a throttle opening of the throttle valve at which the internal pressure in the intake-air passage becomes the basic pressure; and
calculating the target intake-valve closure timing based on the target intake-air quantity when the second control mode is selected.